1. Field of the Invention
The present invention relates to a symbol reading device, symbol reading method and program recording medium to read a symbol of a barcode or two dimensional code where focus is controlled based on size of a captured symbol.
2. Description of the Related Art
Symbols are widely used to identify various pieces of information. A barcode is a symbol representing numeric value information according to specification by one dimensional graphic representation. Lately, a symbol called two dimensional code which holds information in both vertical and horizontal directions is becoming widespread.
A symbol reading device such as a “handy terminal” (portable terminal for commercial use) images an object including a symbol with an imaging device such as a camera and by analyzing the imaged image with a decoding engine, the symbol reading device reads contents of the symbol.
When a symbol such as a barcode or two dimensional code is imaged by a symbol reading device including an imaging device such as a digital camera to extract an ID, it is difficult to find a suitable composition or distance to capture the symbol.
Conventionally, in a digital camera including a seamless focus adjustment function, as a method for selecting the most suitable focus position for capturing an object such as a barcode, the following methods could be used. For example, there is a method of directly measuring a distance to the object with a ranging sensor which uses PSD (Position Sensitive Detector) or by a phase difference method which uses a pair of linear image sensors or measuring a distance by a ranging method using laser irradiation and directly moving the lens to the focus position according to the value.
Since the above described method consumes only about an instant to determine the best focus, a good response can be provided to the user and there is also a merit of high accuracy. On the other hand, a ranging device (sensor) separate from a camera module is necessary, and thus there is the demerit that the cost increases and the size becomes large. Therefore, such method is often used in a single lens reflex camera where there is relatively allowance for cost and size.
Also, other than the method described above, there is a contrast method as a method often used in compact digital cameras, etc.
The contrast method is a method in which images are sequentially captured while moving the focus position from the farthest point to the closest point to find the focus position where the contrast of the captured image is judged to be highest. This method uses the operation of sequentially capturing images while moving the focus position and thus there is a demerit that the method is time consuming.
For example, Japanese Patent Application Laid-Open Publication No. 2006-25055 discloses a technique where an imaging condition setting section is provided to set the imaging condition corresponding to normal photography and barcode capturing and when the barcode is captured, a suitable imaging condition is set as the imaging condition for barcode capturing with the barcode image fitted in the frame for alignment.